Does it void my warranty?
It is very similar to CHDK, which has a FAQ: Does using the CHDK program void your warranty?:
First, please bear in mind that CHDK comes with no warranty for any use; you use it at your own risk. What follows is just a few thoughts, and is not a legal opinion. Many believe that using the CHDK does not harm your warranty, since it is said to be loaded into the memory only temporarily (turn off the cam and it's gone completely) and that it leaves the original camera firmware untouched
They include an email exchange with Canon:
After researching this software on the internet it appears that CHDK doesn't make any actual changes to your camera. If you delete the CHDK software from your memory card, or if you choose not to activate the CHDK software on the card (or remove and replace the batteries), then the camera will behave absolutely normally - nothing has been (or ever is) changed, so the warranty is not affected.
The Magic Lantern firmware is distributed with NO WARRANTY and NO GUARANTEES are provided. It might work. It might not. It hasn't destroyed any cameras yet, but who knows.